Renewable energy from the sun has great potential in reducing dependency on fossil fuels while also providing a cleaner, non-green-house-gas-producing method of power generation. A basic limitation of solar power, however, is its high cost relative to other energy sources. Decreasing the cost per watt can be made possible by improving efficiencies and by decreasing the manufacturing costs of solar cells. Efficiency gains can be realized by increasing the percentage of the solar spectra that is captured and by decreasing loss mechanisms due to charge carrier thermalization, electron/hole recombination, and resistive contact losses. Decreasing solar cell manufacturing costs can also be made possible by utilizing less costly solar cell substrate materials and increasing manufacturing throughput through area device fabrication.
Although the theoretical solar conversion efficiency is 66%, traditional single junction solar cells have a maximum efficiency of only 33%, and in practice, rarely achieve efficiencies greater than 18%. Tandem cells, comprising a multiple stack of junctions where each junction is optimized for progressively longer wavelengths, have achieved significantly higher efficiencies but at even higher costs. Semiconductor quantum dot (QD) based solar cells are ideally suited to increase conversion efficiencies because they have size and compositionally tunable bandgaps and broadband absorption. They are also ideally suited to decrease fabrication costs because they can be deposited over large area planar and nonplanar substrates using low cost spin coating or roll-to-roll processes. “Multi Junction” solar cells comprising layers of quantum dots each with a varying size and/or composition can potentially achieve the greater efficiencies of a tandem cell but at far lower processing costs. Multiple exciton generation (MEG) in lead sulfide quantum dots supplied by Evident Technologies can create more than one electron hole pair per absorbed photon provided that the photon energy is more than twice that of the quantum dot bandgap. By harnessing the MEG process the efficiency loss through charge carrier thermalization could be mitigated and high efficiency solar conversion realized.
Research on quantum dot based solar cells has been ongoing for some time, but it has yet to result in high efficiencies. The vast majority of the research efforts have focused on implementing the nanocrystal colloids into polymer (MEH-PPV, polythiophene, PFO, etc.) solar cells where the quantum dots are either dispersed within a semiconductor polymer, between semiconductor polymer layers, or between a semiconductor polymer layer and an electrode. The interband states present at the inorganic QD/organic polymer interface and QD/polymer band offset result in significant charge carrier recombination, which causes loss of efficiency. The efficiency loss is exacerbated by the organic surfactant layer that envelops colloidal quantum dots. Surfactants enable the particles to disperse in solution, co-solvate with the conjugated polymer, and deposit as a film from solution. Typical surfactants can include TOPO, alkane thiols, and aliphatic amines, all of which are insulators. Any charge transfer from the QDs to the surrounding polymer within solar devices is accomplished through a highly inefficient tunneling process that limits overall device efficiency.
Semiconductor nanocrystals, otherwise known as quantum dots, are tiny crystals typically made of II-VI, III-V, IV-VI, and I-III-VI materials that have a diameter between 1 nanometer (nm) and 20 nm. In the strong confinement limit, the physical diameter of the nanocrystal is smaller than the bulk excitation Bohr radius causing quantum confinement effects to predominate. In this regime, the nanocrystal is a 0-dimensional system that has both quantized density and energy of electronic states where the actual energy and energy differences between electronic states are a function of both the nanocrystal composition and physical size. Larger nanocrystals have more closely spaced energy states and smaller nanocrystals have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric properties of nanocrystals can be tuned or altered simply by changing the nanocrystal geometry (i.e. physical size).
Precise control over nanocrystal size, shape, composition, and surface chemistry allows for the rational engineering of amorphous (i.e. random distribution of colloidal particles within the solid) or crystalline (spatially ordered array of nanocrystals) nanocrystal based colloidal solids. Nanocrystals can be defined by their composition, size, shape, and surface chemistry. All nanocrystals, including semiconductor quantum dots are inherently insoluble without the presence of organic capping molecules referred to as ligands. Ligands have two functional chemical groups, one of which coordinates to the metal atoms comprising the surface of the quantum dot and the other which allows for the nanoparticles to disperse within a given solvent. The strength at which the ligands bind to the nanocrystal surface is dependent on the chemical potential between metal atom and the specific metal coordinating group while the compatibility with a given solvent is dependent upon the magnitude of the polarity or ionization of the opposing moieties. Common metal coordination groups can include phosphine, phosphine oxide, amine, carboxyl, and thiol groups.
Efforts to improve the quality and complexity of assembled colloidal nanocrystals continue to this day. The ability to produce a vast number of nanostructured thin film metamaterials derived from binary populations of colloidal nanocrystals where the particles were self assembled into ordered crystalline lattices has been demonstrated. It has been shown that many types and compositions of nanoparticles can be used including semiconductor quantum dots, several forms of metal nanoparticles, and oxides. By altering the ratio of diameters and the relative concentrations of nanoparticles comprising the nanostructured film many different crystal structures can be formed. Nanostructured layers comprised of single or binary populations of nanoparticles where all the constituents have the same or nearly the same diameter can pack neatly into a hexagonal close packed structure, whereas if the ratio of diameters and/or the prevalence of the various constituents is changed from 1:1 continuously to 1:13 cubic, orthorhombic, and tetragonal symmetries are produced. It has also been found that dipole-dipole and van der Waals forces play a significant role in the symmetries produced.
The limitations of present quantum dot/polymer solar cells, LEDs, and other optoelectronic devices result from energy transport inefficiencies at the organic/inorganic interface. These detrimental effects can be largely mitigated by employing all quantum dot based nanostructured thin layers that are devoid of organic materials in the active region of the optoelectronic devices.